The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. Various methods may be used to enhance the productivity of fluids from wells formed in subterranean formation, such as hydrocarbon-producing wells that produce oil or gas. Different characteristics or properties of the wells may limit the production of fluids. These may include insufficient flow paths in the formation, wellbore coatings and near-wellbore formation damage resulting from prior treatments or operations, such as from drilling fluids and the like, that limit fluid flow.
One method of treating such wells to enhance production involves the use of acids or acid-based fluids for dissolving portions of the formation to create alternate flow paths and for removing wellbore coatings and near-wellbore formation damage. Such acids or acid-based fluids are useful for this purpose due to their ability to dissolve both formation minerals and contaminants, such as those that were introduced into the wellbore/formation during drilling or remedial operations and which may coat the wellbore or have penetrated the formation. In the case of treatments within the formation, rather than wellbore treatments, the portion of the formation that is near the wellbore and that first contacts the acid is usually adequately treated. Portions of the formation further from the wellbore, however, may remain untreated by the acid, due to the acid reacting before it can penetrate very far from the wellbore.
Carbonate formations and materials are well suited for treatment with acids because they readily dissolve in a variety of different acids. Sandstone or siliceous formations or materials, however, are only susceptible to dissolution in hydrofluoric (HF) acid. Thus, sandstone formations are often treated with a mixture of hydrofluoric and hydrochloric acids (called mud acid). This acid mixture is often selected because it will dissolve clays (found in drilling mud) as well as the primary constituents of naturally occurring sandstones (e.g., silica, feldspar, and calcareous material). Such treatments may be carried out at low injection rates to avoid fracturing the formation.
A major problem with sandstone acidizing as presently practiced is that multiple stages are required to prevent deleterious reactions between components of the formation or dissolved materials and the acidizing fluids. Of particular concern is contact between dissolved calcium ions and fluoride ions that can produce a solid fluorite (CaF2) that can partially negate the effectiveness of the treatment.
Chelating agents can be used to keep calcium and other metal ions in solution to prevent precipitation of these solid compounds. The chelating agents may have limited solubility, however, in low pH fluids. This presents a particular problem as a higher pH may make the acidizing fluid less effective.
Because of these problems, several stages of fluids are typically required in acid treatment of sandstone formations. These stages include 1) a brine stage (e.g. KCl or NH4Cl) to displace incompatible cations, such as Ca2+ and Na+ ions, away from the wellbore; 2) an acid stage (e.g. HCl, organic acid, etc.) to dissolve or remove the calcium or magnesium carbonate in the zone to prevent precipitation of CaF2; 3) a primary silicate dissolution stage (e.g. HF+HCl or an organic acid) to remove alumino-silicates; and 4) a final brine stage to displace the dissolved ions and spent acids away from the critical matrix. FIG. 1 illustrates a prior art multiple stage treatment. In particular, the calcium removal stage and the silicate dissolution stages both alter the permeability of the matrix and thus affect the injection of subsequent fluids.
Because placement and proper diversion facilitate successful treatment of the formation, having multiple stages may make optimal placement difficult, as each stage may ultimately be positioned differently. What is therefore needed is a means for effectively treating sandstone or siliceous formations or materials with acid or acid-based solutions that reduces the amount of precipitates formed, in particular CaF2 precipitates, and that eliminates the need for multiple steps and treatments, which can result in improper placement and inadequate treatment.